The mailing process involves a number of operations including assembly and insertion of mail items, e.g., documents and inserts, into an envelope, moistening the envelope flap, sealing the envelope, weighing the envelope and applying postage. Automation of such operations, for example in an inserting machine, typically entails moving the mail items or envelopes through a plurality of stations, each station carrying out one of the operations of the process. With respect to envelope flap moistening and sealing operations, see for example, U.S. Pat. Nos. 1,194,568 (Storck), 2,749,689 (Colley), 4,233,931 (Gingerich et al.), 4,371,416 (Denzin), 4,428,794 (Hayskar et al.) and 4,450,037 (Gavronsky).
Typically, the moistening operation includes the introduction of water into an automatic envelope moistening apparatus that includes a reservoir, means in communication with the reservoir for applying moisture to the flap, and means for adding water to the reservoir. The fluid level in the reservoir must be maintained above a minimum level to ensure that a suitable amount of moisture is applied to the flap. In some cases, visual observation of the water in the reservoir may be possible if the reservoir tank is transparent or contains a transparent window which allows observation of the water level. In other cases, the reservoir is situated in the moistening apparatus such that observation is not possible. In general, even when the water level is observable, some form of fluid level detection is required to prevent the water level inadvertently falling below the minimum level.
A variety of fluid level detection methods are known. Those methods suitable for use in a moistening apparatus have encountered problems which are inherent in the particular method or are the result of improper maintenance. For example, one method detects water level by measuring the conductivity in the water. This method has had problems when there is a variance in the conductivity of the water. Such a variance can occur when an operator adds distilled water to the reservoir, causing a lower conductivity of the water in the reservoir. This results in the sensors failing to detect the fluid level accurately. In another scenario, distilled water is required for use in the reservoir because less maintenance is needed to keep the moistening apparatus operating properly. The sensors are calibrated for sensing the conductivity of the distilled water. The sensors fail to detect the fluid level accurately when the operator adds tap water to the reservoir.
Other level detection methods include optical sensor arrangements in which light beams pass through the reservoir and the fluid. One such method includes a float which interrupts the light beam from detection by the sensors. Such optical systems are susceptible to contamination of algae or scum or other material in the reservoir, thus requiring cleaning the reservoir at regular intervals to maintain the integrity of the fluid level detection.
Another level detection methods includes a float with an arm connected to a sensor outside the reservoir. Any such float and arm arrangement requires a high profile reservoir so that the arm has sufficient room to maneuver to ensure proper sensing of fluid level.
A fine resolution level detector is disclosed in U.S. Pat. No. 4,589,282 issued to Dumery on May 20, 1986. This detector includes a large number of equally spaced Hall sensors which are mounted in a tube to form a column of sensors. An annular float containing a compound magnet assembly is slidably fitted about the tube. The compound magnet assembly generates a pattern of regularly spaced apart magnetic field regions wherein the pattern of Hall sensor output voltage provides unique vernier measure of liquid level which is added to a rough measure obtained from the count of the Hall sensor closest to the float. It is clear that Dumery's detector is suitable for detecting fluid level when a fine resolution of fluid level is required. However, Dumery's detector is excessive for detecting only low and high levels in a reservoir.
In small moistening devices, such as in a table top inserting machine, there is a desire to have a reservoir that has a low profile but in which low or high fluid level can be reliably detected. Space requirements of such a low profile reservoir do not facilitate the use of the aforementioned methods of level detection.